The antagonism between the essential divalent metals calcium and magnesium and the divalent metal carcinogens, lead, nickel and cadmium are under investigation in metabolic and in carcinogenicity studies. Magnesium and calcium completely prevented lung adenoma production in strain A mice by nickel and lead acetates. Magnesium, but not calcium, prevented the formation of injection site sarcomas and delayed the development of testicular carcinomas in rats receiving cadmium.